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  • 1.
    Andersson, Sebastian
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Thermoelectrically Controlled Spin-Switch2010In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 46, no 6, p. 2140-2143Article in journal (Refereed)
    Abstract [en]

    The search for novel spintronic devices brings about new ways to control switching in magnetic thin-films. In this work we experimentally demonstrate a device based on thermoelectrically controlled exchange coupling. The read out signal from a giant magnetoresistance element is controlled by exchange coupling through a weakly ferromagnetic Ni-Cu alloy. This exchange coupling is shown to vary strongly with changes in temperature, and both internal Joule heating and external heating is used to demonstrate magnetic switching. The device shows no degradation upon thermal cycling. Ways to further optimize the device performance are discussed. Our experimental results show a new way to thermoelectrically control magnetic switching in multilayers.

  • 2. Benbouzid, M. E. H.
    et al.
    Reyne, G.
    Meunier, G.
    Kvarnsjo, L.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Dynamic modelling of giant magnetostriction in Terfenol-D rods by the finite element method1995In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 31, p. 1821-1824Article in journal (Refereed)
    Abstract [en]

    As a contribution to the development of methods for the design and the analysis of devices based on giant magnetostrictive materials, a model for the simulation of the dynamic behaviour of the nonlinear magnetoelastic medium is presented. The coupled magnetic, magnetoelastic and mechanical equations that describe the magnetostrictive problem are solved by means of the finite element method. The thin sheets bending principle (surface splines) is used to introduce in the simulation the nonlinear properties of giant magnetostrictive materials, obtained by static characterizations

  • 3. Bergqvist, A.
    et al.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    A stress-dependent magnetic Preisach hysteresis model1991In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 27, p. 4796-4798Article in journal (Refereed)
    Abstract [en]

    The authors present a generalization of the classical Preisach model which handles coupled magnetic and mechanical hysteresis. Magnetostrictive materials are known to have hysteresis with respect to both magnetic field H and mechanical stress lambda;. To test the validity of the model, experiments where the two components H and lambda; have been verified in many different ways have been performed on Terfenol-D and compared to results computed from the model. Some of these results are presented. This stress-dependent model is found to have an accuracy comparable to that of the classical Preisach model

  • 4. Bergqvist, A.
    et al.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    A thermodynamic representation of pseudoparticles with hysteresis1995In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 31, p. 3539-3541Article in journal (Refereed)
    Abstract [en]

    A method of expressing pseudoparticles with hysteresis within a context of irreversible thermodynamics is investigated. The state of a pseudoparticle is uniquely determined by its magnetization and its evolution is governed by entropy maximization. Hysteresis appears if the free energy is a nonconvex function of magnetization. The vectorial nature of magnetization and dependence on rate are accounted for in a systematic manner. Some basic properties are derived for quasistatic processes. In particular, it is found that in the scalar case, the magnetization is a monotonically increasing functional with respect to field and that for quasistatic processes, this implies the wiping-out property

  • 5. Bergqvist, A. J.
    et al.
    Engdahl, Göran
    KTH, Superseded Departments, Electrical Systems.
    A homogenization procedure of field quantities in laminated electric steel2001In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 37, no 5, p. 3329-3331Article in journal (Refereed)
    Abstract [en]

    A method for representing structures of laminated electric steel is investigated. The average behavior of laminates, including eddy current effects, is approximately represented using a rate-dependent constitutive law. The accuracy of the method is studied by comparing the homogenized description with finite difference calculations.

  • 6.
    Bissal, Ara
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Magnusson, Jesper
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Comparison of two Ultra-fast actuator concepts2012In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 48, no 11, p. 3315-3318Article in journal (Refereed)
    Abstract [en]

    In this paper, two different types of ultra-fast electromechanical actuators are compared using a multi-physical finite element simulation model that has been experimentally validated. They are equipped with a single-sided Thomson coil (TC) and a double-sided drive coil (DSC), respectively. The former consists of a spirally-wound flat coil with a copper armature on top, while the latter consists of two mirrored spiral coils that are connected in series. Initially, the geometry and construction of each of the actuating schemes are discussed. Subsequently, the theory behind the two force generation principles are described. Furthermore, the current, magnetic flux densities, accelerations, and induced stresses are analyzed. Moreover, mechanical loadability simulations are performed to study the impact on the requirements of the charging unit, the sensitivity of the parameters, and evaluate the degree of influence on the performance of both drives. Finally, it is confirmed that although the DSC is mechanically more complex, it has a greater efficiency than that of the TC.

  • 7.
    Bissal, Ara
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Salinas, Ener
    Magnusson, Jesper
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    On the Design of a Linear Composite Magnetic Damper2015In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 51, no 11, article id 8003305Article in journal (Refereed)
    Abstract [en]

    High-voltage direct current (HVdc) breakers are the key components in the realization of multiterminal HVdc grids. In the presence of fault current, these breakers should be able to deliver impulsive forces to swiftly open the metallic contacts. After the acceleration phase, the moving armature should be decelerated using controllable forces to avoid plastically deforming fragile components integrated in the system. In this paper, finite-element method-based simulation models, complimented with small-scale and large-scale experimental prototypes, were utilized to benchmark different damping topologies. It was found that a Halbach-based configuration can deliver a damping force that is almost two and a half times larger than its sequel. Its sequel, composed of vertically stacked oppositely oriented magnets, is easier to assemble and is also capable of generating a considerable damping force. Finally, it has been shown that both these schemes, inserted in a composite tube, have a potential to be used as dampers in HVdc breakers.

  • 8. Bjurman, U.
    et al.
    Engdahl, Göran
    KTH, Superseded Departments, Electrical Systems.
    Separation of high and low frequency behavior in non-linear circuit simulation, excited with modulated signals2000In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 36, no 4, p. 968-970Article in journal (Refereed)
    Abstract [en]

    Exciting nonlinear circuits with modulated signals will generate crossmodulation frequencies. In ordinary simulation techniques this makes time stepping methods a necessity, In the case of EMC-testing where the carrier frequency is as high as 2 GHz and the modulating frequency only 1 kHz, the simulation time will be extremely long. This paper presents Time Domain Frequency Separation (TDFS), a method,where the high and low frequency behavior is calculated separately to decrease the computation time. In a simulation made with Saber the calculation took approximately fourteen hours, and with TDFS in Matlab two minutes.

  • 9.
    Bormann, Dierk
    et al.
    KTH, School of Electrical Engineering (EES).
    Tavakoli, Hanif
    KTH, School of Electrical Engineering (EES).
    Reluctance Network Method for Calculating the Series Impedance Matrix of Multi-Conductor Transmission Lines2013In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 49, no 10, p. 5270-5279Article in journal (Refereed)
    Abstract [en]

    The theoretical foundation of a recently proposed reluctance network method for computing the complex series impedance matrix of multi-conductor transmission lines is presented in detail, and the method is extended to more general cross section geometries with gaps of non-constant width between the conductors. It is argued that the method becomes exact in the limit of high frequencies and narrow gaps between the conductors. This limit usually is the most difficult one in alternative approaches, especially when the proximity effect is concerned. The method is verified by comparison with the exact solution of a stacked-slabs geometry, and with finite-element field calculations on a geometry consisting of tightly packed, round wires, surrounded by a shield.

  • 10.
    Bormann, Dierk
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Tavakoli, Hanif
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Reluctance Network Treatment of Skin and Proximity Effects in Multi-Conductor Transmission Lines2012In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 48, no 2, p. 735-738Article in journal (Refereed)
    Abstract [en]

    A novel method is developed to easily compute the complex series impedance matrix of amulti-conductor transmissionline in a wide frequency range. It is based on a network of complex reluctances describing flux paths around the conductors. The network parameters are derived from the geometry of the conductor-insulator arrangement and from its material parameters. The method is best suited for situations where the typical gap width between conductors is much smaller than their thicknesses, where it provides an accurate treatment of both skin and proximity effects. Especially the proximity effect is hard to treat by other means in that limit. The method is demonstrated for an idealized example geometry, where its accuracy is verified with harmonic FEM field calculations.

  • 11.
    Cherepov, Sergiy
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Konovalenko, Alexander
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Worledge, D. C.
    IBM T.J. Watson Researh Center.
    Micromagnetics of Spin-Flop Bilayers: S, C, and Vortex Spin States2010In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 46, no 6, p. 2124-2127Article in journal (Refereed)
    Abstract [en]

    Spin-flop tunnel junctions subjected to strong gigahertz excitations are found to exhibit highly stable resistance states, intermediate between the two spin-uniform ground states of high and low resistance. The associated spin distributions are necessarily nonuniform and differ significantly from the ground-state anti-parallel spin configuration in their static and dynamic properties. Detailed micromagnetic modeling reveals that inplane spin vortices in dipole-coupled thin-film bilayers are stable spin configurations, where the orientation of the vortex cores and the vortex chirality play an important role in the response of the system to external magnetic fields.

  • 12.
    Cherepov, Sergiy
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Worledge, D. C.
    IBM T.J. Watson Researh Center.
    Resonant Switching of Two Dipole-Coupled Nanomagnets2010In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 46, no 6, p. 2112-2115Article in journal (Refereed)
    Abstract [en]

    The storage layer of recently developed spin-flop magnetic random-access memory consists of two closely spaced dipole-coupled nanomagnets and is highly stable in the ground state as well as in quasistatic fields applied off the easy axis. We show experimentally and confirm by using micromagnetic simulations that these spin-flop bilayers can be switched relatively easily by dynamic fields, applied at the frequency of the optical spin resonance of the bilayer. The field amplitude sufficient for this resonant switching can be an order of magnitude lower than the fields necessary for quasistatic reversal. Our data and micromagnetic analysis suggest that thermal agitation can play a role in the observed resonant switching behavior.

  • 13. Dave, Renu W.
    et al.
    Steiner, G.
    Slaughter, J. M.
    Sun, J. J.
    Craigo, B.
    Pietambaram, S.
    Smith, K.
    Grynkewich, G.
    DeHerrera, M.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Tehrani, S.
    MgO-based tunnel junction material for high-speed toggle magnetic random access memory2006In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 42, no 8, p. 1935-1939Article in journal (Refereed)
    Abstract [en]

    We report the first demonstration of a magnetoresistive random access memory (MRAM) circuit incorporating MgO-based magnetic tunnel junction (MTJ) material for higher performance. We compare our results to those of AlOx-based devices, and we discuss the MTJ process optimization and material changes that made the demonstration possible. We present data on key MTJ material attributes for different oxidation processes and free-layer alloys, including resistance distributions, bias dependence, free-layer magnetic properties, interlayer coupling, breakdown voltage, and thermal endurance. A tunneling magnetoresistance (TMR) greater than 230% was achieved with CoFeB free layers and greater than 85% with NiFe free layers. Although the TMR with NiFe is at the low end of our MgO comparison, even this MTJ material enables faster access times, since its TMR is almost double that of a similar structure with an AlOx barrier. Bit-to-bit resistance distributions are somewhat wider for MgO barriers, with sigma about 1.5% compared to about 0.9% for AlOx. The read access time of our 4 Mb toggle MRAM circuit was reduced from 21 ns with AlOx to a circuit-limited 17 ns with MgO.

  • 14. Dumas, Randy K.
    et al.
    Redjai Sani, Sohrab
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Mohseni, S. Majid
    Iacocca, Ezio
    Pogoryelov, Yevgen
    Muduli, Pranaba K.
    Chung, Sunjae
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Durrenfeld, Philipp
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. Univ Gothenburg, Sweden.
    Recent Advances in Nanocontact Spin-Torque Oscillators2014In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 50, no 6, p. 4100107-Article in journal (Refereed)
    Abstract [en]

    We present a comprehensive review of the most recent advances in nanocontact spin torque oscillators (NC-STOs). NC-STOs are highly tunable, with both applied magnetic field and dc, broadband microwave signal generators. As opposed to the nanopillar geometry, where the lateral cross section of the entire device has been confined to a typically <100 nm diameter, in NC-STOs, it is only the current injection site that has been laterally confined on top of an extended magnetic film stack. Three distinct material combinations will be discussed: 1) a Co/Cu/NiFe pseudospin valve (PSV) where both the Co and NiFe have a dominant in-plane anisotropy; 2) a Co/Cu/[Co/Ni](4) orthogonal PSV where the Co/Ni multilayer has a strong perpendicular anisotropy; and 3) a single NiFe layer with asymmetric non-magnetic Cu leads. We explore the rich and diverse magnetodynamic modes that can be generated in these three distinct sample geometries.

  • 15. Dumas, Randy K.
    et al.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Fang, Yeyu
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Bonanni, Valentina
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Lau, June W.
    Nogués, Josep
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Graded Anisotropy FePtCu Films2011In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 47, no 6, p. 1580-1586Article in journal (Refereed)
    Abstract [en]

    The fabrication and subsequent analysis of continuously graded anisotropy films are discussed. During deposition, a compositional gradient is first achieved by varying the Cu concentration from Cu-rich (Fe53Pt47)(70)Cu-30 to Cu-free Fe53Pt47. The anisotropy gradient is then realized after thermal post-annealing, and by utilizing the strong composition dependence of the low-anisotropy (A1) to high-anisotropy (L1(0)) ordering temperature. The magnetic properties are investigated by surface sensitive magneto-optical Kerr effect and alternating gradient magnetometer (AGM) measurements. AGM first-order reversal curve (FORC) measurements are employed in order to provide a detailed analysis of the reversal mechanisms, and therefore the induced anisotropy gradient. At low annealing temperatures, the FORC measurements clearly indicate the highly coupled reversal of soft and hard phases. However, significant interdiffusion results in virtually uniform films at elevated annealing temperatures. Additionally, the A1 to L1(0) ordering process is found to depend on the film thickness.

  • 16.
    Engdahl, Göran
    et al.
    Power Engineering, ABB Corporate Research, Västerås.
    Lerebourg, B.
    Ecole Nationale SupCrieure d’Hydraulique et de Mkcanique de Grenoble, St Martin d’ Hères.
    Experimental study of magnetic slurries regarding magnetic properties1996In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 32, no 5, p. 5109-5111Article in journal (Refereed)
    Abstract [en]

    The aim of this study was to assess the basic magnetic properties of a mixture composed of a commercial lubricant and soft magnetic iron particles, where the particle content is significantly higher than in conventional magnetic fluids, due to the fact that the particles verge on the sedimentation level. This so called magnetic slurry can, then, be regarded as a highly ductile magnetic conducting material for tentative use in a magnetic circuit that can change shape under operation. The high particle content results in high saturation field densities that extend the application field towards electric power engineering. This paper, therefore, focuses on properties like relative permeability, losses, the Q-factor and the efficiency for the power frequencies 50 and 400 Hz. It shows that the relative permeabilities are low (below 10). The values of the efficiency and the Q-factor decrease considerably above 0.5 T. That limits the application of the studied slurry in electric power engineering. In such applications, the powder properties must be improved

  • 17. Engel, B. N.
    et al.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Butcher, B.
    Dave, R. W.
    DeHerrera, M.
    Durlam, M.
    Grynkewich, G.
    Janesky, J.
    Pietambaram, S. V.
    Rizzo, N. D.
    Slaughter, J. M.
    Smith, K.
    Sun, J. J.
    Tehrani, S.
    A 4-mb toggle MRAM based on a novel bit and switching method2005In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 41, no 1, p. 132-136Article in journal (Refereed)
    Abstract [en]

    4-Mb magnetoresistive random access memory (MRAM) with a novel magnetic bit cell and toggle switching mode is presented. The circuit was designed in a five level metal, 0.18-mum complementary metal-oxide-semiconductor process with a bit cell size of 1.55 mum(2). The new bit cell uses a balanced synthetic antiferromagnetic free layer and a phased write pulse sequence to provide robust switching performance with immunity from half-select disturbs. This switching mode greatly improves the operational performance of the MRAM as compared to conventional MRAM. A detailed description of this 4-Mb toggle MRAM is presented.

  • 18.
    Engquist, Björn
    et al.
    KTH, School of Computer Science and Communication (CSC), Numerical Analysis, NA.
    Greenbaum, A
    Murphy, W.D.
    Global boundary conditions and fast Helmholtz solvers1989In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 25, no 4, p. 2804-2806Article in journal (Refereed)
    Abstract [en]

    Electromagnetic scattering from a conducting two-dimensional cylinder is modeled by solving Helmholtz's equation with the far-field radiation boundary condition replaced by a global boundary condition allowing. This allows the boundary condition to be applied very near the scatterer. The discrete problem is solved by a biconjugate gradient algorithm. IncompleteLUdecomposition is used as a preconditioning strategy, resulting in very fast convergence. The numerical solution is compared with known solutions and found to converge fasters forka&les;10, wherekis the wave number andais the radius of the cylinder

  • 19.
    Gromov, Andrey
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Gigahertz Sandwich Strip Inductors Based on Fe-N Films: The Effect of Flux Closure at the Flange2010In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 46, no 6, p. 2097-2100Article in journal (Refereed)
    Abstract [en]

    Planar strip inductors consisting of soft Fe-N films enclosing a conductor made of Cu are fabricated on Si substrates and exhibit 70 to 100% inductance gain at 1 Ghz with quality factors of 3 to 5. The magnetic part of the inductance is less than predicted theoretically, which is attributed to hardening of the magnetic material at the edges of the strip, where the deposition is close to 60 degrees incidence. Test films were fabricated on tilted substrates and found to develop very high anisotropy for deposition angles larger than 30 degrees. Optimizing the flux closure at the strip edges is essential for further improving the performance of sandwich strip inductors.

  • 20. Heinonen, O. G.
    et al.
    Muduli, P. K.
    Iacocca, E.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Decoherence, Mode Hopping, and Mode Coupling in Spin Torque Oscillators2013In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 49, no 7, p. 4398-4404Article in journal (Refereed)
    Abstract [en]

    Spin torque oscillators (STOs) often exhibit multiple modes, leading to complex behavior. One example is mode hopping between different eigenmodes of a magnetic tunnel junction (MTJ) STO. This mode hopping is a strong function of current and angle between the magnetization in the free and fixed layers, and away from anti-parallel configuration, mode hopping can be the dominant decoherence process. Another example is the linewidth of a nanocontact STO that can be a complex non-monotonic function of temperature in regions where two or more modes are excited by the oscillators. These phenomena require a generalization of the single-mode nonlinear STO theory to include mode coupling. We derive equations describing the slow time evolution of the coupled system and show they describe a dynamically driven system, similar to other systems that exhibit mode hopping in the presence of thermal fluctuations. In our description, mode coupling also leads to additional coupling between power and phase fluctuations, which can in certain limited cases lead to longer relaxation times for power fluctuations, and consequently to larger linewidths through the nonlinear frequency shift.

  • 21. Hollmark, M.
    et al.
    Tkatch, V. I.
    Grishin, Alexander M.
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Khartsev, Sergiy
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Processing and properties of soft magnetic Fe40Co40P14B6 amorphous alloy2001In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 37, no 4, p. 2278-2280Article in journal (Refereed)
    Abstract [en]

    The Fe40Co40P14B6 metallic alloy has been prepared in a glassy state by a melt-spinning process. The 15-25 mum thick as-quenched, ribbons have superior soft magnetic properties compared with those of well-known Fe40Ni40P14B6 glass: the saturation magnetization is about 1.45 T, coercive field 4.0 A/m @ 0.1 Hz, incremental magnetic permeability mu (max) similar to 90 000 @ 60 Hz and Curie temperature above 700 K. The crystallization temperature, determined by differential scanning calorimeter, is approximately 60 K higher than that for FeNi-based alloy, indicating the enhanced thermal stability of FeCo-glass.

  • 22.
    Holmgren, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Bodarenko, Artem
    KTH. Institute of Magnetism, National Academy of Science, 03142 Kiev, Ukraine.
    Koop, Björn
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Ivanov, Boris
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Non-Degeneracy and Effects of Pinning in Strongly Coupled Vortex Pairs2017In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 53, no 11, article id 4400505Article in journal (Refereed)
    Abstract [en]

    We study the effects of pinning on the quasi-static behavior of stacked, strongly coupled spin-vortex pairs in magnetic multilayered nanopillars, with vertical vortex separation small compared with the vortex-core size. The small separation causes the core-core interaction to be the dominant energy contribution for small applied fields and excitations, which results in highly non-linear dynamics. The properties of such a vortex pair are expected to only be dependent on the relative vortex core polarizations and relative chiralities, so that the individual configurations should be degenerated. We show how pinning can lift this degeneracy, which can be used to distinguish the individual chirality configurations.

  • 23. Huang, Xinghui
    et al.
    Nagamune, Ryozo
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.).
    Horowitz, Roberto
    A comparison of multirate robust track-following control synthesis techniques for dual-stage and multisensing servo systems in hard disk drives2006In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 42, no 7, p. 1896-1904Article in journal (Refereed)
    Abstract [en]

    This paper presents the system modeling, design, and analysis of multirate robust track-following controllers for a dual-stage servo system with a microelectromechanical systems (MEMS) microactuator (MA) and an instrumented suspension. A generalized model is constructed which includes a nominal plant, disturbances, uncertainties, and multirate sensing and control. Two major categories of controller design methodologies are considered. The first includes synthesis methodologies that are based on single-input single-output (SISO) design techniques, and includes the sensitivity decoupling (SD) and the PQ methods. In this case, a high sampling-rate inner loop damping control is first implemented using the auxiliary sensor signals. Subsequently, a low-rate outer loop controller is designed for the damped plant using either the SD or PQ design methods. The second category of design methodologies includes those based on multirate, multi-input multi-output (MIMO) design techniques, including mixed H-2/H-infinity, mixed H-2/mu, and robust H-2 synthesis. In this case, a set of controllers, which is periodically time-varying due to multirateness, is designed by explicitly considering plant uncertainty and hence robust stability. Comparisons are made between all the design techniques in terms of nominal H-2 performance, robust stability, and robust performance between these controllers, When the feedback controller is closed around the full order, perturbed plant. The advantages and disadvantages of each of these methods are discussed, as well as guidelines for their practical implementation.

  • 24.
    Iovan, Adrian
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Lam, Kanber
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Andersson, Sebastian
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Cherepov, Sergiy
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Haviland, David B.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Tunneling spectroscopy of magnetic double barrier junctions2007In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 43, no 6, p. 2818-2820Article in journal (Refereed)
    Abstract [en]

    Scanning tunneling microscopy (STM) is used to study transport in magnetic double tunnel junctions (DTJs) formed using a fixed transparency barrier of a patterned tunnel junction (TJ), and a variable tunnel barrier between the top electrode of the patterned junction and the STM tip. A sufficiently thin top electrode has been predicted to result in a rectification of charge current through a DTJ when the two barriers have different transparency. Our measurements indeed show a high current rectification ratio for 3-nm-thick, continuous film top electrodes, which is observed for junctions with asymmetric tunnel barriers.

  • 25.
    Jalali-Roudsar, Amir A.
    et al.
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Denysenkov, Vasyl
    KTH, Superseded Departments, Physics.
    Khartsev, Sergey
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Grishin, Alexander
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Adachi, N.
    Okuda, T.
    Microwave and magneto-optic properties of pulsed laser deposited bismuth iron garnet films2001In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 37, no 4, p. 2454-2456Article in journal (Refereed)
    Abstract [en]

     We report on processing and comparative characterization of epitaxial Bi3Fe5O12 (BIG) films grown by pulsed laser ablation of a precursor oxide target onto (NdGd)(3)(ScGa)(5)O-12 [NGSGG(111)] and Gd-3 (ScGa)(5)O-12 (GSGG(001)] single crystals. Comprehensive X-ray diffraction analyses reveal the epitaxial quality of the BIG films: they are single phase, exclusively (111) and (001) oriented with less than 0.4 degrees and 0.06 degrees of the full width at half maximum of the rocking curve of main texture Bragg reflection [(111) for NGSGG and (001) for GSGG substrate, respectively]. The films are strongly in-plane textured with cube-on-cube film-to-substrate epitaxial relationship. VSM and ferromagnetic resonance measurements revealed in-plane magnetization in BIG/GSGG(001) film, while the BIG/NGSGG(111) film was found to have perpendicular magnetization. For BIG(001) and (111) films the saturation magnetization 4 piM(s) was found to be 1400 and 1200 Gs; the Faraday rotation at 635 nm was -7.8 and -6.7 deg/mum; the constant of uniaxial magnetic anisotropy was K-u* = -8.70 X 10(4) and +1.16 x 10(4) erg/cm(3); the constant of cubic magnetic anisotropy K-1 = -3.6 x 10(3) and -7.14 x 10(3) erg/cm(3). High Faraday rotation and low coercive field (less than or equal to 40 Oe) of BIG/GSGG(001) films show promise for their use in integrated magneto-optic applications.

  • 26. Kahl, S.
    et al.
    Grishin, Alexander M.
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Khartsev, Sergiy
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Kawano, K.
    Abell, J. S.
    Bi3Fe5O12 thin film visualizer2001In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 37, no 4, p. 2457-2459Article in journal (Refereed)
    Abstract [en]

    Micrometer thick Bi-3 Fe-5 O-1.2 (BIG)(001) films were grown onto Gd-3(ScGa)(5)O-12(001) single crystals by the pulsed laser deposition technique. The films are of epitaxial quality with a lattice constant of 1.263 nm. Narrow x-ray rocking curves (full width at half maximum: 0.06 deg) and narrow ferromagnetic resonance lines (widths of 25 Oe) indicate good crystalline quality and magnetic performance. The films possess in-plane magnetization, Faraday rotation as high as -7.8 deg/mum at 630 nm wavelength, and the magnetization M-H curves are linear in the field range from -1.6 kOe to +1.6 kOe. The magneto-optical (MO) properties are almost temperature independent between 5 K and 200 K. This enables the use of thin BIG films as visualizers of magnetic fluxes in high-T-c superconductors. Magnetic fluxes have been visualized in YBa2Cu3O7-infinity, (YBCO) ceramics with an artificial grain boundary. The sensitivity expressed as Faraday rotation angle (per film thickness) per magnetic field unit was found to be 4.9 deg/(kOe . mum) in a 730 nm thick BIG film.

  • 27.
    Koop, Björn C.
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Dzhezherya, Y. I.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Quantitative magnetic characterization of synthetic ferrimagnets for predictive spin-dynamic behavior2014In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 50, no 11, article id 6971569Article in journal (Refereed)
    Abstract [en]

    Geometric or magnetic asymmetries in synthetic antiferromagnetic particles give rise to ferrimagnetic-like magnetization behavior, both quasi-static and dynamic. Such asymmetries in synthetic ferrimagnets can originate from a thickness imbalance or a fringing field from the reference layer in a nanopillar stack. In this paper, we theoretically describe the effects of the corresponding magnetic asymmetry contribution on the structure's static and spin-dynamic behavior. The developed model is then used to experimentally determine the asymmetry parameters of typical nanoscale spin-flop junctions, as well as successfully describe their microwave resonant properties, such as the frequency splitting and the field dependence of the optical spin-resonance spectra.

  • 28.
    Koop, Björn C.
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Ivanov, B. A.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Nonlinear dynamics in spin vortex pairs with strong core-core coupling2014In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 50, no 11, article id 6971321Article in journal (Refereed)
    Abstract [en]

    We investigate the dynamics of spin vortex pairs in magnetic multilayer particles, with the vortices closely spaced vertically and therefore with strong core-core coupling. We focus on the spin-dynamic behavior of the system beyond the linear small-signal regime, and on the state with antiparallel vortex chiralities and parallel cores, in which the vortex cores are strongly dipole coupled. The data show a clear transition from the dominant single rotational resonance at 2-3 GHz for small excitation field amplitudes to a dominant gyrational resonance at high excitation fields. The concomitant changes in the microwave spectra, seen as satellite resonances near the rotational peak as well as a pronounced low-frequency resonance, are interpreted as arising from the nonlinearities of the main rotational mode, which also mediate microwave power transfer from the high- to the low-frequency mode.

  • 29.
    Koop, Björn
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Descamps, Thomas
    KTH.
    Holmgren, Erik
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Relaxation-Free and Inertial Switching in Synthetic Antiferromagnets Subject to Super-Resonant Excitation2017In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 53, no 11, article id 4300505Article in journal (Refereed)
    Abstract [en]

    Applications of magnetic memory devices greatly benefit from ultra-fast, low-power switching. In this paper, we propose a method for how this can be achieved efficiently in a nano-sized synthetic antiferromagnet by using perpendicular-to-the-plane picosecondrange magnetic-field pulses. Our detailed micromagnetic simulations, supported by analytical results, yield the parameter space where inertial switching and relaxation-free switching can be achieved in the system. We furthermore discuss the advantages of dynamic switching in synthetic antiferromagnets and, specifically, their relatively low-power switching as compared with that in single ferromagnetic particles. Finally, we show how the excitation of spin waves in the system can be used to significantly reduce the post-switching spin oscillations for practical device geometries.

  • 30.
    Korenivski, V.
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Leuschner, R.
    Thermally Activated Switching in Nanoscale Magnetic Tunnel Junctions2010In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 46, no 6, p. 2101-2103Article in journal (Refereed)
    Abstract [en]

    Magnetic tunnel junctions 90 to 300 nm wide and of aspect ratio approximate to 2 are studied using high-speed pulse fields with regard to the soft-layer magnetization reversal under thermal agitation. It is found that the larger cells, 200-300 nm wide, reverse through nonuniform magnetization states with the energy barriers to thermal activation an order of magnitude smaller than those expected for single-domain magnets. The single-domain limit is reached for the smallest cells, having elliptical soft layers approximately 90 nm wide and 150-200 nm long. The magnetization decay in the small cell limit is well described by the Stoner-Wohlfarth single-domain model and the Arrhenius activation law. The results demonstrate that the penalty due to the smaller magnetic volume is compensated by a larger relative energy barrier to activation as the junction size is reduced to similar to 90 nm. This determines the important length scale for geometric scaling of such technologies as magnetic random access memory.

  • 31.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    RDesign of magnetic transceivers for 3-d integrated circuits2014In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 50, no 11, article id 6971254Article in journal (Refereed)
    Abstract [en]

    Use of magnetic film inductors as transceiver elements for wireless transfer of power and signal communication between vertically stacked silicon integrated circuits is discussed. Two characteristic designs are compared - a vertical magnetic core with high-aspect-ratio laminations and a planar magnetic core where flux closure is achieved by a suitable design of the interchip air gap. The latter design is argued to hold a promise for 3-D circuit integration.

  • 32. Krah, J. H.
    et al.
    Bergqvist, A.
    Engdahl, Göran
    KTH, Superseded Departments, Electrical Systems.
    Homogenization algorithms for calculation of field quantities in laminated magnetic materials2002In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 38, no 5, p. 2385-2387Article in journal (Refereed)
    Abstract [en]

    A recently suggested homogenization procedure for the calculation of field quantities in laminated electric steel has proved to considerably speed up the calculation time. One limitation of this method is that it only works in situations where the skin effect is small. This paper presents some approaches to extend the method such that substantial skin effects can be treated.

  • 33.
    Krah, Julius Hartwig
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Optimum discretization of a physical Cauer circuit2005In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 41, no 5, p. 1444-1447Article in journal (Refereed)
    Abstract [en]

    physical Cauer circuit is used to model skin effect in a conductor or eddy currents in a magnetic lamination. A physical lumped element model was chosen, because equivalent circuits are inconvenient for nonlinear materials such as magnetic materials. Approaches to discretize the used circuit are presented and compared. An iterative fitting technique is suggested that requires fewer elements and yields higher accuracy than an exponential discretization.

  • 34.
    Krings, Andreas
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Mousavi, Seyedali
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Wallmark, Oskar
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Soulard, Juliette
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Temperature Influence of NiFe Steel Laminations on the Characteristics of Small Slotless Permanent Magnet Machines2013In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 49, no 7, p. 4064-4067Article in journal (Refereed)
    Abstract [en]

    High performance electrical machines can operate at temperatures of 100 degrees C and beyond in rotor and stator cores. However, magnetic properties are generally measured at room temperatures around 23 degrees C to 25 degrees C according to the standards, even if it is known that the magnetization of some materials is substantially influenced by increasing temperatures. This paper investigates the thermal influence on the magnetic properties and iron losses in the stator cores of small slotless permanent magnet synchronous machines (PMSMs). The stator stack is made of thin nickel iron (NiFe) lamination sheets. Magnetic measurements of the stator core are conducted for different frequencies and flux densities at several temperatures between 25 degrees C and 105 degrees C. The obtained measurement data is afterwards used in finite element method (FEM) simulations to investigate the influence of the magnetic property change on the machine performance. For the PMSM in consideration, the FEM simulations show that an increased stator core temperature reduces the electromagnetic torque considerably; approximately 1/3 of the torque reduction due to increased rotor magnet and stator core temperatures (from 25 degrees C to 100 degrees C) can be attributed to the increased stator core temperature.

  • 35. Kvarnsjo, L.
    et al.
    Bergqvist, A.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    A method of measuring eddy current impedances in giant magnetostrictive materials1990In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 26, p. 2574-2576Article in journal (Refereed)
    Abstract [en]

    A method of calculating eddy-current impedances for giant magnetostrictive materials during dynamic magnetization is suggested. The method is based on solving the power-balance equation for each time step of the recorded data. The experimentally obtained impedances can be used for accurately modeling the magnetostrictive process

  • 36. Kvarnsjo, L.
    et al.
    Bergqvist, A.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Application of a stress-dependent magnetic Preisach hysteresis model on a simulation model for Terfenol-D1992In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 28, p. 2623-2625Article in journal (Refereed)
    Abstract [en]

    A previously reported simulation model for the highly magnetorestrictive material. Terfenol-D has been extended by implementation of the recently developed stress-dependent magnetic Preisach hysteresis model. The model is based on static measurements of material characteristics combined with equations for the dynamic mechanical and magnetic behavior of the material. As a test of the model the authors have simulated the behavior of a Terfenol-D rod for different load conditions. The results show that the model is able to describe the influence of nonlinearities, eddy currents, and hysteresis on the magnetomechanical transduction process of magnetorestrictive materials

  • 37. Kvarnsjo, L.
    et al.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    A set-up for dynamic measurements of magnetic and mechanical behavior of magnetostrictive materials1989In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 25, p. 4195-4197Article in journal (Refereed)
    Abstract [en]

    A setup for dynamic studies of giant magnetostrictive rods has been developed. The mechanical load on the magnetostrictive rod can be electronically controlled, which makes it possible to study the magnetomechanical behavior in complex situations. Computer simulations show that the setup can be used as a tool for designing magnetomechanical devices. The setup is described, including the mechanical arrangement, sensors, current feed, and data acquisition, and its preliminary operation and verification are discussed

  • 38. Kvarnsjo, L.
    et al.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Nonlinear 2-D transient modeling of Terfenol-D rods1991In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 27, p. 5349-5351Article in journal (Refereed)
    Abstract [en]

    A nonlinear model for simulation of the transient behavior of magnetostrictive Terfenol-D rods has been developed. The model is based on static characterization of the magnetic and mechanical properties of the rods combined with Maxwell’s equations and classical mechanic laws. Registrations for different rods are stored in a database which is called upon by the model. The model is able to handle longitudinal wave propagation and eddy current influence under transient conditions. Any combination from the set of magnetic field (current), magnetic flux density (voltage), mechanical motion, mechanical stress, and mechanical impedance can be used as the two independent variables in the model. This modeling technique implies that the experimental set-up used for data acquisition of the material properties should be regarded as a part of the modeling process. The experimental set-up used for the characterization of Terfenol-D properties can produce magnetic fields and mechanical stresses independently of each other and of arbitrary shape. A specially designed sample holder provides a homogeneous magnetic field and mechanical stress field inside the rod

  • 39. Liu, Y. Q.
    et al.
    Bondeson, A.
    Bergstrom, R.
    Johnson, Claes
    Larson, M. G.
    Samuelsson, K.
    Eddy-current computations using adaptive grids and edge elements2002In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 38, no 2, p. 449-452Article in journal (Refereed)
    Abstract [en]

    Results are presented from eddy-current computations using adaptive techniques, based on rigorous a posteriori error estimates. The adaptivity restores the quadratic convergence with grid size of the magnetic energy, despite singularities occurring at corners. A new procedure is introduced to satisfy the solvability condition for the curl-curl equation. The methods are applied to a model of a hydrogenerator, with anisotropic conductivity and permeability. The ungauged formulation with both vector and scalar potentials gives very significant improvements in rate of convergence for this problem. Reasons for the improved convergence are discussed.

  • 40. Losin, C.
    et al.
    Gomez-Polo, C.
    Knobel, M.
    Grishin, Alexander M.
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Torsional dependence of second-harmonic amplitude of giant magnetoimpedance in FeCoSiB amorphous wire2002In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 38, no 5, p. 3087-3089Article in journal (Refereed)
    Abstract [en]

    The effect of torsional stresses in the magnetoimpedance response of a (Co0.94Fe0.06)(72.5)Si12.5B15 amorphous wire is analyzed. In particular, the occurrence of a second-harmonic contribution of the magnetoimpedance voltage is analyzed and compared with the first-harmonic amplitude (impedance). The evolution of both harmonic components are correlated to the estimated permeability contributions obtained through the analysis of the estimated rotational hysteresis loops. The highest torsional dependence of the second-harmonic contribution can be used in the development of new magnetoelastic sensors.

  • 41. Lundgren, A.
    et al.
    Tiberg, H.
    Kvarnsjo, L.
    Bergqvist, A.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    A magnetostrictive electric generator1993In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 29, p. 3150-3152Article in journal (Refereed)
    Abstract [en]

    An electric generator based on the magnetostrictive effect is presented. Longitudinal oscillations of a Terfenol-D rod give rise to a varying flux which induces a current in a coil wound around the rod. A small prototype of such a device is discussed, and both calculations and experiments are performed. The problem of eddy current losses is addressed

  • 42.
    Masood, Ansar
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Tamaki, Takahiko
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ström, Valter
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Rao, K. Venkat
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    A New Class of Materials for Magneto-Optical Applications: Transparent Amorphous Thin Films of Fe-B-Nb and Fe-B-Nb-Y Metallic Glassy Alloys2014In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 50, no 4, p. 4004005-Article in journal (Refereed)
    Abstract [en]

    Optically highly transparent, soft magnetic thin films (4-18 nm thick) of Fe-B-Nb- and Fe-B-Nb-Y-based glassy metal targets were grown on quartz substrates by pulsed laser deposition, and their optical and magneto-optical properties were investigated over the visible spectrum (400-700 nm). All the films found to be fully amorphous in structure were continuous with uniform thickness and surface morphology. Their optical transmittance in the range 50%-85% was found to be film thickness dependent over the entire visible regime. The Verdet constant (V) and Faraday rotation angle (theta(f)) for different films (similar to 4-18 nm) investigated as a function of wavelength (lambda) show considerably higher values for the films of Fe-B-Nb-Y alloy as compared with those for Fe-B-Nb films, e. g., the similar to 4 nm film of Fe-B-Nb-Y alloy exhibits V similar to 49 degrees/Oe cm and theta(f)similar to 26 degrees/mu m while it decreased to similar to 29.4 degrees/Oe and similar to 11.8 degrees/mu m, respectively, for the Fe-B-Nb alloy at lambda=611 nm. A linear relationship is found for the wavelength dependence of V and theta(f) for both alloy systems. To the best of our knowledge, these values are considerably higher than those reported for any other magneto-optic material. The films are found to be soft magnetic with a high saturation moment while their magnetic coercivity values increases with thinness of the films. The observed combination of optical and magneto-optical properties of this new class of amorphous metallic films makes them viable for multifunctional magneto-optical applications.

  • 43. Meessen, K. J.
    et al.
    Thelin, P.
    Soulard, Juliette
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Lomonova, E. A.
    Inductance calculations of permanent-magnet synchronous machines including flux change and self- and cross-saturations2008In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 44, no 10, p. 2324-2331Article in journal (Refereed)
    Abstract [en]

    Accurate inductance calculation of permanent-magnet synchronous machines is a relevant topic, since the inductances determine a large part of the electrical machine behavior. However, the inductance calculation, as well as the inductance measurement, is never a completely straightforward task when saturation occurs. In this paper, the total flux in the d and q axes are obtained from finite-element method or measurements and therefore include saturation and cross-couplings. The inductances are obtained from analytical post-processing based on an equivalent magnetic circuit. The originality of this method is that it accommodates the changes in the magnet flux and the inductances with the level of saturation. The resulting inductance values are the ones seen by the converter or the grid, as found by a more accurate approach.

  • 44.
    Mousavi, Seyedali A.
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Differential Approach of Scalar Hysteresis Modeling Based on the Preisach Theory2011In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 47, no 10, p. 3040-3043Article in journal (Refereed)
    Abstract [en]

    This paper introduces a differential approach to model scalar hysteresis based on the Preisach theory. This model can generate hysteresis trajectories for both symmetrical and asymmetrical minor loops and keep the memory of a material in a proper manner. It can easily be inverted, it has a simple algorithm with suitable computational speed, and is not memory consuming. Also, the obtained results of the model are in very good agreement with measurements.

  • 45.
    Muduli, Pranaba
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Pogoryelov, Ye.
    Mancoff, F.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Modulation of Individual and Mutually Synchronized Nanocontact-Based Spin Torque Oscillators2011In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 47, no 6, p. 1575-1579Article in journal (Refereed)
    Abstract [en]

    We study current modulation of spin torque oscillators (STOs) based on single and double nano-contacts. For single nano-contact devices, we show modulation for a carrier frequency of 19-40 GHz and modulation frequencies from 50 MHz to 1 GHz. We show that the behavior of modulation and the asymmetry in the power of the modulation sidebands can be very well modeled, and predicted, using nonlinear frequency and amplitude modulation (NFAM) calculations based on the free-running STO parameters. Modulation of synchronized double nano-contact STOs is equally well modeled by NFAM theory and demonstrates the feasibility of communication applications based on arrays of synchronized STOs.

  • 46.
    Nguyen, Thi Ngoc Anh
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. Vietnam Natl Univ, Vietnam.
    Fallahi, Vahid
    Le, Quang Tuan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Chung, Sunjea
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Mohseni, Seyed Majid
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. Shahid Beheshti Univ, Iran.
    Dumas, Randy K.
    Miller, Casey W.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. Univ Gothenburg, Sweden.
    Investigation of the Tunability of the Spin Configuration Inside Exchange Coupled Springs of Hard/Soft Magnets2014In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 50, no 6, p. 2004906-Article in journal (Refereed)
    Abstract [en]

    Magnetic multilayer (ML) structures comprising a perpendicular magnetic anisotropy (PMA) layer coupled to an in-plane magnetic anisotropy (IMA) layer are promising materials for zero/low field operating spin-torque oscillators and bit patterned recording media. The magnetization tilt angle can be easily tuned by varying the IMA layer thickness due to the competition between PMA and IMA layers. To explore the underlying magnetization reversal mechanism and to further understand the control of tilt angle and uniformity of the magnetization, the IMA (NiFe, Co, and CoFeB)/PMA (Co/Pd MLs) exchange spring systems are systematically studied. Experimental data obtained from magnetometry show good agreement with 1-D micromagnetic simulations, allowing us to design tunable exchange coupled spring as a function of IMA thickness.

  • 47.
    Norgren, Martin
    et al.
    KTH, Superseded Departments, Electrical Systems.
    He, Sailing
    KTH, Superseded Departments, Electromagnetic Theory.
    Exact and explicit solution to a class of degaussing problems2000In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 36, no 1, p. 308-312Article in journal (Refereed)
    Abstract [en]

    An exact and explicit solution to a class of degaussing problems is given. The solution is shown to be unique. The stability of the solution is also analyzed.

  • 48. Owens, Frank J.
    et al.
    Gupta, Amita
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Rao, K. Venkat
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Iqbal, Zafar
    Guillen, J. M. Osorio
    Ahuja, R.
    Guo, J. -H
    Unusual room temperature ferromagnetism in bulk sintered GaP doped with copper2007In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 43, no 6, p. 3043-3045Article in journal (Refereed)
    Abstract [en]

    Robust room temperature ferromagnetism is obtained in single phase Gallium Phosphide doped with Cu2+ prepared by simple solid state reaction route. The saturation magnetization at 300 K is 1.5 x 10(-2) emu/g and the coercivity was found to be 125 Oe. A strong ferromagnetic resonance signal confirms the long range magnetic order which persists to temperatures as high as 739 K. X-ray absorption spectroscopy (XAS) indicate that Cu is in a +2 state. Ab initio calculations also show that the ferromagnetic ordering is energetically favorable in Cu doped GaP. When the spin-orbit coupling is included we get an enhanced total magnetic moment of 0.31 mu(B) with a local moment on Cu 0.082 and on P 0.204 mu(B). per atom.

  • 49. Parra, Erick O. Burgos
    et al.
    Bukin, Nick
    Dupraz, Maxime
    Beutier, Guillaume
    Sani, Sohrab Redjai
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Popescu, Horia
    Cavill, Stuart A.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. Univ Gothenburg, Sweden.
    Jaouen, Nicolas
    Keatley, Paul S.
    Hicken, Robert J.
    van der Laan, Gerrit
    Ogrin, Feodor Y.
    Holographic Magnetic Imaging of Single-Layer Nanocontact Spin-Transfer Oscillators2016In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 52, no 7, article id 6500304Article in journal (Refereed)
    Abstract [en]

    Time-averaged images of the magnetization within single-layer spin-transfer oscillators have been obtained using the holography with extended reference by autocorrelation linear differential operator technique. Transport measurements on a Pd(5)-Cu(20)-Ni81Fe19(7)-Cu(2)-Pd(2) (in nanometers) stack with a 100 nm diameter nanocontact reveal the presence of vortex dynamics. Magnetic images of the device for injected current values of 24 and -24 mA suggest that a vortex has been ejected from the nanocontact and become pinned at the edge of the region that is visible through the Au mask.

  • 50.
    Persson, Johan
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Sani, Sohrab Redjai
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Bonetti, Stefano
    Magnusson, Fredrik
    Pogorylov, Yevgen
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Mohseni, Seyed Majid
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Gunnarsson, Sten
    Norling, Martin
    Stoij, Christer
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Spin-Torque Oscillator in an Electromagnet Package2012In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 48, no 11, p. 4378-4381Article in journal (Refereed)
    Abstract [en]

    Spin-torque oscillators (STO) hold promise for multi-octave frequency operation at very high frequencies and modulation speeds. STO operation is typically demonstrated using large electromagnets and probe stations, and has so far not been packaged with a portable form factor. For STOs to be utilized in real applications, a smaller packaging solution is needed. We integrate STOs with packages originally developed for YIG oscillators, modified to incorporate permanent magnets and to achieve a compact portable oscillator based on the STO.

12 1 - 50 of 74
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